Prolongations of the Fundamental Structure (Introduction)
The Three Blind Mice example demonstrates how almost the whole nursery rhyme can be understood as a prolongation of just one part of the fundamental structure - the over I.

The diminution that prolongs is a tonic arpeggiation and, although it is decorated in the foreground by neighbour notes and consonant skips, it does not offer any opportunity for extensive prolongation.

In other words , it is easy to see how a very short piece can be generated from the fundamental structure, but how about longer pieces?

Schenker suggests that particular forms of the basic diminutions already discussed (plus a number of other devices) are especially suitable for generating large sections of music. When these diminutions are prolonging notes of the fundamental structure they are known as the first level middleground.

The example below shows a descending third progression. This, along with a descending fifth from and a descending third from are the only descending linear progressions found in the first level middleground.

For progressions closer to the foreground of a piece of music the rules are gradually relaxed. Schenker suggests that the rules of Species Counterpoint are essentially about coherence - counterpoint that breaks these rules does not make sense.

In a real piece of music, progressions in the first level middleground have to be so strict because they support large sections of the music. Diminutions closer to the surface of the music can rely on the coherence of the middleground progressions they prolong to help them make sense.

Schenker was very particular about which progressions can appear in the first level middleground for the same reasons - only some prolongations of the fundamental structure are robust enough to support large spans of music.